Vertical, horizontal, and discursive channels of subnational diffusion: institutional isomorphism and local ordinances supporting borderline intellectual functioning in Korea

How do newly visible vulnerable groups enter subnational legislative agendas in centralised unitary states, and why does recognition spread quickly without advocacy infrastructure, professional consensus, or a central mandate? This study examines the early diffusion of local ordinances supporting persons with borderline intellectual functioning (BIF) across 243 Korean general-purpose local governments in 2016–2023, a window covering the innovator and early-adopter stages of an ongoing process. Drawing on institutional isomorphism, we estimate panel logit and event history models of coercive (upper-tier adoption), mimetic (peer adoption share), and normative pressure, measured through national media coverage and local non-profit density. Upper-tier adoption, peer imitation, and media salience accelerated adoption, whereas non-profit density and partisan variables had no effect. The results identify a boundary condition of the normative channel: where professional communities have not yet organised, media discourse carries normative pressure. In unitary systems, vertical cues dominate even when horizontal imitation should be strongest.